Firecuda 530 only gets 3100MBs speed on 2019 Mac Pro.

That Sabrent card has THE most confusing name there is. It has x16, x8, and x4 in its name AND in it connectors.

But it only make contact with x4 worth of contacts in the slot. It's only an x4 card, despite its name.

Cards that are wider are typically in the US$200 and up range.

cards that provide full x16 capabilities at the speed of the SSDs tend to cost upwards of US$800 bare, higher with SSDs included. Take a look at this independent test:

https://barefeats.com/owc-accelsior-8m2.html

comparative Prices are graphed in to last graph..

https://www.owc.com/solutions/accelsior-8m2 from US$799

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The reason I had some of these answers close at hand is that I tried a FireCuda in an 4x card on a Mac Pro 5,1, that has only PCIe 2.0 slots. To my great dismay, it was only able to achieve 700-ish out of the 7000-ish M Bytes/sec advertised speed. I knew I would not see 7,000, but I hoped a really fast SSD would produce something good.

I only bought the US$20-ish card to put it in, but it turns out that specific FireCuda SSD in its current form does not downshift properly for PCIe 2.0, and if I used a different SSD in an x4 card like the Sabrent or the OWC 1M2, I get 1400-ish speeds.

But it must be a bug, because if you move those same x4 cards to an x16 slot and change nothing else, the FireCuda steps up to the expected 1400-ish M Bytes/sec. I sent my results to Seagate, but the entire process of dealing with them was torturous. They are not geared up for users to identify bugs and send them bug reports. Most of their support appears to be done from outside the USA.

This just in:

https://www.sonnettech.com/product/m2-4x4-pcie-card/overview.html from US$400

appears to be a new x16 card